Individuals typically seek out, or are referred to, physical therapists (PT) for rehabilitation under one of three situations: 1) they are experiencing muscle or joint pain, 2) they have recently undergone surgery and need to regain a function lost due to the surgery, or 3) they have injured themselves at work or during recreation and need to restore function while limiting the risk of re-injury. Restoration of muscle function is by far the largest component of most rehabilitation protocols. This includes restoring range of motion, muscle strength and joint stability.
Individuals also seek out trainers in order to improve their performance, generally with the goal of enhancing performance without increasing the risk of injury or re-injury. Training focuses on three interrelated aspects of muscle function: strength, coordination, and bursting power or impulse.
In both scenarios, therapists and trainers prefer to use training activities that mimic the functional activities of the subject, and to evaluate function by accurately assessing the condition of the musculoskeletal system during such activities.
Range of motion is readily quantified through the use of inexpensive, easy to use, goniometer measurements.
Muscle strength measurements, by contrast, have posed more significant challenges. This, in large part, is because existing means for evaluating how strong a muscle is provide no direct measurements of the muscle's physiological activity. Instead, measurements of the displacement of a mass or resistance to an applied force are relied upon as surrogate indices of a muscle's functional status. Evaluation is typically made through specific performance measures (throwing, hitting, kicking, etc.), since no means exist for evaluating, in real-time in the field environment, the force-generating capabilities of a muscle. In these surrogate types of measurements, whether the muscle is lengthening or shortening, how fast the muscle is moving, the position of the limb or body part associated with the muscle(s) of interest, etc., are uncontrolled variables, any or all of which may influence the outcome of the measurement.
One means of measuring muscle strength is the dynamometer, either laboratory based or hand-held. Dynamometers measure the torque produced by a muscle, or group of muscles, at a skeletal joint, not muscle strength per se. Isokinetic dynamometers minimize shortening artifacts but are extremely expensive to purchase and maintain, and require trained personnel to operate. While hand-held dynamometers are less expensive and are easier to use, they are limited by the strength of the individual performing the measurement. In no case can a dynamometer measurement be made during typical functional activities (i.e. under conditions that physical therapists and trainers refer to as “closed chain” activities). Of most consequence, dynamometers can record the effect of only one muscle group at a time. They cannot separate out the contributions of each of the several muscles that cooperate to provide the complementary, supplementary, and/or antagonistic actions required to execute a particular motion. Physical therapists and trainers, however, need such differentiable information if they are to design safe and reliable exercises by ensuring that the strength of the involved muscles is well-balanced. As a result of these many limitations, dynamometers are used only very rarely in the PT clinic or among trainers. Consequently, PTs and trainers typically have no objective means for accurately and reproducibly evaluating muscle strength.
Joint stability, especially dynamic joint stability, cannot be measured objectively with available technology. The skills and experience of the examiner must be relied upon extensively. Attempts to quantify dynamic joint stability by measuring “joint laxity” (e.g., U.S. Pat. No. 4,649,934) confront the limitation that only passive stabilization can be employed in such measurements, which provides measurements for only a fraction of the components of dynamic stabilization.
Muscle coordination reflects the timing and strength of contraction of multiple muscles during a function. Again, evaluation is typically made through specific performance measures (throwing, hitting, kicking, etc.).
Bursting power is sometimes evaluated through dynamometer measurements, though it is more usually evaluated through a specific performance measure (e.g., height of a jump or exercise repetitions in a given time period).
The foregoing methods are inadequate for determining the effectiveness of a training program or for motivating an athlete/patient by providing objective feedback on a specifiable training goal. They are also less than acceptably reliable for evaluating whether an athlete/patient has safely reached a performance level equal to the risk of the competition/work that the individual intends. Finally, their effectiveness in medical diagnosis/prognosis is marginal.
More effective means of assessing muscle strength, coordination and joint stability are needed. Assessment should be founded on a reproducible, real-time, quantitative measure of effort expended by specific muscles of an individual during voluntary and stimulated contractions, whether concentric, eccentric and/or isometric, through the entire range of motion. The measurement should be realizable with portable, inexpensive testing apparatus, and should be of a form that permits a valid comparison of the effort of the various muscles involved in accomplishing a functional closed chain activity, either within one individual, or between individuals.